Faculty & Staff Profiles

Section 8 - Retaining Structures, Earthwork Contracts, Soil Reports

Retaining Structures - Lecture Notes

Retaining walls are usually built to hold back soil mass. However, retaining walls can also be constructed for aesthetic landscaping purposes.

Figure 1

There are three types of retaining walls:I. Simple Gravity Wall - These walls usually consist of large blocks of concrete or poured concrete. They are stabilized simply by the weight of the wall.II. Cantilever Wall - These walls are many times made of concrete. They use the weight of the backfill to help keep the wall stable. This type of wall is shown in the diagram above. It is important to build this type of wall strong enough to withstand substantial internal stresses where the stem and base are connected.III. Tie-back Wall - These walls can be composed of a variety of different materials (sheet piling is popular). They are stabilized by tying parts of the stem to a cable or metal rod. The cable or rod is then connected to an "anchor" that is buried deep and far back into the backfill. It is important to keep the anchor far enough away from the wall so that it is outside the radius of the most probable slip surface within the backfill.(Some walls can be combinations of these three types.)

Design of the wall needs to be made with the type of backfill in mind. Clayey soils are poor backfill material because of the large lateral pressures they may exert and the poor drainage characteristics. Many times, the backfill just to the right of the retaining wall (shown with dashed curves in the diagram above) consist of sand and gravel surrounding a drainage tile.

The design of the wall must:1. Resist sliding along its base.2. Resist overturning.3. Not exceed the bearing capacity of the soil beneath the base.4. Avoid excessive settlement.5. Built structurally strong to resist failure from the build up of internal stresses produced by external forces. (But this is true for any structure!)

Well-built wall near Jarvis Hall at UW-Stout.

Poorly built retaining wall on 21st Street in Menomonie, WI.

Primer on Working With Distributed Loads

Consider the following example of a farm wagon loaded with corn.

Figure 2

To analyze the forces and pressures acting on the wagon, we need to first determine the pressures, P1 and P2.

P1 = gh = (7.4 kN/m3)(0.6m) = 4.44 kN/m2

Likewise, P2 = 7.4 kN/m2. Now, the pressure needs to be decomposed into a component that is uniform across the wagon bed and a part that is changing with position.

Figure 3

The F/L vector in part (A) is calculated by

F/L = P1(length) = (4.44 kN/m2)(8 m) = 35.5 kN/m.

The number 35.5 kN/m represents the force per length (in and out of the paper) acting in the center of the wagon. The total force acting on the wagon, due to the uniform load, is

This number, 11.8 kN/m, represents the force per length (in and out of the paper) acting at a point 1/3 the distance from the high pressure side (or right side). Therefore, the total force acting on the wagon, due to the distributed load, is

(F/L)(lengthin/out) = (11.8 kN/m)(6 m) = 70.8 kN.

We are now left with the force diagram:

Figure 4

We have just determined the magnitude of the red vectors, now to determine the magnitude of the blue vectors (representing the wheels supporting the wagon), we need to apply the conditions for static equilibrium. First of all, the sum of all the forces must add to zero. Thus, 0 = F1+ F2 - F3 - F4. Secondly, the sum of all the torques (or moments) tending to twist the wagon bed about the pivot point is also equal to zero. This second condition will enable us to determine one of the blue vectors (assuming clockwise to be positive):

Solving this equation, gives us F1 = 128 kN. Applying the first condition of equilibrium,

0 = F1+ F2 - F3 - F4 = 128 kN + F2 - 213 kN - 70.8 kN,

solving this equation gives us F2 = 155.8 kN. The problem is solved. A force of 128 kN is shared equally between the two front tires (or 64 kN per tire) and a force of 155.8 kN is shared equally between the back tires (or 77.9 kN per tire).

So how is this related to the forces acting on a retaining wall? Just rotate the wagon bed 90o clockwise, and you'll pretty much have a situation that is very similar to the forces acting on a retaining wall.

In section 6 it was mentioned that the lateral pressure (or horizontal pressure) that develops within soil as a function of depth is about 1/2 the value of the vertical pressure, or PL=0.5PV. It is now time to "refine" this analysis by making the multiplication factor a variable that is dependent upon the soil type. Such that, PL=Ko(PV), where Ko is defined as the coefficient of earth pressure at rest. So what does earth pressure at rest mean? Earth pressure at rest refers to the lateral pressure caused by earth (or soil) that is prevented from lateral movement by an unyielding wall.

Representative Values of Ko

Soil Type

Ko

Granular, loose

0.5-0.6

Granular, dense

0.3-0.5

Clay, soft

0.9-1.1 (undrained)

Clay, hard

0.8-0.9 (undrained)

Soil can exert active and passive pressures. To get an idea of what is meant by these two terms, consider a frictionless (between backfill and wall), infinitely rigid wall that is allowed to slide. The soil is allowed to expand in the lateral direction. Shearing resistance developed within the soil mass because of the soils shear strength acts opposite to the direction of the expansion. Thus, a soil's cohesion helps to reduce the lateral pressures applied to a wall that is allowed to move. There has been several theories put forth that takes into consideration active and passive earth pressures. In these cases, Ko becomes Ka and is now a function of the angle of internal friction, cohesion, vertical pressure, and some geometrical parameters.

Examples of determining forces and pressures on some simple retaining walls

Some popular types of retaining walls

Gabion retaining walls consist of heavy gauge wire boxes that enclose large diameter rocks (the rocks are called rip-rap). The boxes are stacked and fastened together. This type of retaining wall is often used for erosion control and soil retainment along river banks. [Picture to the right is a gabion retaining wall built along the Snake River in Idaho. The wall has a hiking trail on top of it.]

(A)Gabion stepped back, cantilever design

(B)Gabion stepped front, simple gravity design

Thinner retaining wall designs that usually consist of sheeting, piling, and/or planks. These often require tiebacks for stability.

Earthwork Contracts - Lecture Notes

Someone who desires something built or constructed is designated the owner. The owner may have consulted with architects, insurance companies, engineers, etc., before formulating any specific designs for a project. The owner will advertise for bids to complete the project. At this point in time, a contractor will put together a proposal to complete the project for a specified cost and submit it as a bid. Once a bid has been chosen, a contract will be written up and signed by both the owner and the contractor.

The relationship between parties involved with a construction project can be schematically illustrated as:

Model 1

In this model, the owner designs and finances the project. The contractor performs the necessary work to complete the project and the engineer works for the owner and oversees the work to ensure the project conforms to the specifications in the contract. The engineer and owner should speak as one voice when interacting with the contractor. The extent of direct interaction between the contractor and engineer may vary depending on the project. For smaller projects an engineer may not be involved.

Another model that describes the parties involved in a project may include a construction manager. This is shown below.

Model 2

This model is very similar to model 1. The construction manager works very closely with the owner, contractor and sub-contractors. They observe the work in progress and manage the materials needed and the contractors involved. In this model, the manager is usually employed by the owner and does not work directly for the contractors.

Contract

Legal aspects of contracts is a bit beyond the scope of this course. But it is important to introduce you to the general content of a contract that you may encounter when the project involves earthwork or excavating. Within such a contract, there should be a section with the heading Earthwork. This section will include

Scope

Materials

Workmanship (or Quality/Tolerance of work)

Payment

Scope describes the project and the work needing to be accomplished. It is a general overview of the earthwork needed.

The Materials are usually categorized as either classified or unclassified. If a material is classified it must conform to a certain specification. An example of this would be when a coarse grained (according to USCS classification) soil is needed for fill-dirt This section may include statements requiring you to perform testing, or submit samples for testing, to confirm that the classified material meets specifications. Soil at the project site or excavated soil at the site should be used as construction material whenever possible to reduce the need of bringing in fill from elsewhere.

Workmanship may involve considerations for blocking traffic (autos, boats, etc.), interrupting water flow or drainage, specific sloping may be needed, etc. This section is just a description of how closely you need to follow design plans and what latitude exist for accomplishing the task in a different way.

Payment for earthwork is usually put forth as a cost per volume of obtaining, moving, and placing (or removal of waste material from the site) both classified or unclassified material. This part may also contain statements regarding who pays for down time. This is the time in which equipment or laborers cannot perform the work because of weather, non-arrival of materials, etc.

Most contracts will contain statements about the subsurface conditions and require the contractor to indicate that they have carefully examined this information, are aware of the conditions that may affect their work, and can perform the work for the bid price. The contract may have extensive information about the subsurface or possiblly little to no information. Before signing such a contract, the bidder may need to perform (or request) more subsurface information to be confident about how much work is required. Most statements, like the ones described above, are in a contract to protect the owner against frivolous or unwarranted extra costs that the contractor may require for completing the project.

On occasion, a contractor may encounter a situation that was unpredictable (by most reasonable standards) and requires additional work and expenses. At this point, the owner and contractor need to discuss this unforeseen situation and come to an agreement on payment for expenses and work. Most disputes that arise between a contractor and owner after the project is begun can be worked out by mutual agreement. In some disputes, the possibility of needing a third party mediator is warranted. The last step in resolving contract disputes involves filing legal claims and pursuing it through the legal system.

Soil Reports - Lecture Notes

"Owner's sometimes forego borings (to save money) and then ask you to make recommendations. This is like asking a medical doctor to make a diagnosis but not allowing the doctor to perform tests."

Bill Kwazny, P.E.

Soil reports are documents that contain information about the subsurface structure and composition. Some reports are quite brief and only contain a limited amount of information. Other reports can be quite elaborate with thousands (maybe millions) of dollars spent to examine the subsurface structure and provide recommendations on how this examination will affect the project being proposed. Sometimes the project may need modified due to subsurface considerations. The most important reason for examining the subsurface is to ensure the structure's foundation will be sufficiently supported. This means the subsurface must have sufficient bearing capacity and will not be subjected to unacceptable settling characteristics. Other reasons might include subgrade working conditions, dewatering, making sure nearby structures are not adversely affected by excavating, geological stability (earthquakes, mass movements, etc.), cost of excavating - digging into solid rock may require a special plan, shoring or sloping considerations, etc.

For massive structures such as suspension bridges and multi-story buildings, a detailed soil engineering report will be financed. These reports usually contain:

Scope and Purpose

Introduction

Geological Setting

Field Studies Performed

Laboratory Tests Performed

Analysis

Conclusions and Recommendations (Including an appendix for a more detailed look at the numbers obtained in the tests.)

The smaller soil reports might only contain information about the water table and a qualititative measure of soil type as a function of depth (such as a SPT) for a limited number sampling positions on a grid.

Two popular soil tests that are often found on reports include the Standard Penetration Test (SPT) and/or the Cone Penetration Test (CPT).

Standard Penetration Test is widely used in the U.S. It is inexpensive, can be quickly performed, and is simple. It consists of a hardened steel, split spoon sampler that is attached to the end of a drilling rod and driven into the ground.

Split spoon sampler for the Standard Penetration Test

This device must conform to a standardized geometric design. It is driven into the ground with a drop hammer that weighs 140 lb and falls 30 in for every hammering. When it is driven 18 in into the ground, the standard penetration resistance (N-value) is the number of blows to move the last 12 in. This device can obtain a sample of the soil as a function of depth. A soil type boundary is encountered when the corrected N-value significantly changes at a particular depth. The N-value as a function of depth needs to be corrected due to overburden pressure. This means that even if the soil type does not change as a function of depth, the non-corrected N-value will gradually go up because of the overburden pressure. And the corrected N-value should remain constant as a function of depth if the soil type does not change.

Mathematical corrections needed:

Ncorrected = CNNmeasured

where CN is the correction factor with the values

CN = 0.77Log10(20/p), if p < 0.25 tsf

or

CN = 0.77Log10(19.5/p), if p > 0.25 tsf.

The p quantity represents the overburden pressure at that depth.

Workers are marking off a split-spoon sampler to determine N-values as a function of depth. This work was being done just North of the Menomonie public library.

Contents of the split-spoon sampler are being extracted, marked, and bagged for laboratory analysis.

The Cone Penetration Test is widely used in Europe and is gaining in popularity in the U.S. It consists of a cone and a friction sleeve that has a standard geometric design as shown below.

Cone and friction sleeve for the Cone Penetration Test

The CPT measures (i) cone resistance, and (ii) cone plus sleeve resistance. One can then mathematically solve for the sleeve resistance by the equation Ffriction = Ftotal - Fcone, where F represents a corrected N-value. The data from this test is usually presented as cone resistance, friction resistance, and friction ratio. The friction ratio is defined as (Ffriction/Fcone). As a general rule, the friction ratio is larger in cohesive soils than in cohesionless soils.

The CPT can be further divided into a Static Cone Test (or Dutch Cone Test) which uses a hydraulic device to drive the cone into the ground. This device is capable of measuring the resistance encountered as a function of depth. The Dynamic Cone Test (another CPT) drives the cone into the ground with a hammer.